This invention relates in general to ultrasonic transducers such as those used for process monitoring, and in particular, provides an ultrasonic transducer assembly for monitoring the quality of spot welds during the welding process.
There exist many industrial manufacturing processes in which accurate monitoring and control of spot weld quality are critical to the structural integrity of the assembly. In the automobile industry, for example, it has been long been recognized that a means of monitoring the quality of spot welds would be of great value.
Prior art systems for monitoring weld quality have attempted to monitor weld quality, while the weld was being created, by passing ultrasonic waves through the welding electrodes and the forming weld by the through-transmission technique. More recently, there have been additional efforts to utilize acoustic emission techniques to monitor weld quality and to control the welding process Pulse-echo ultrasonic techniques are presently being utilized to evaluate weld quality after the weld is created, but these techniques have associated deficiencies in evaluating weld thickness due to variable weld geometry and interpretation.
Recently, there have been attempts to construct an ultrasonic transducer for monitoring spot weld quality that can be fitted into a commonly used spot weld electrode. In one prior art approach, a ring transducer is fitted on the inside shoulder of a weld electrode. This design yields a poor signal due to interaction of the ultrasound energy with the walls of the electrode. Additionally, this design fails to concentrate ultrasound energy on the centerline of the electrode where the weld nugget forms.
A second prior art approach to utilizing ultrasound for monitoring spot weld quality involves bonding a miniature transducer assembly to the inside surface of the electrode. This technique yielded good ultrasonic signal characteristics. However, apparatus constructed in accordance with this prior art approach has a useful life of only approximately 60 weld cycles.
There are two reasons for the premature failure of devices constructed according to this latter prior art approach. First, spot welding electrodes are commonly constructed of pure copper. This material is exceedingly difficult to prepare properly for bonding, due to the rapid formation of thin, loosely adherent oxide layers on the surface of the copper. Secondly, as welds are generated, the weld electrode heats up. Although weld electrodes are generally water cooled, it is estimated that the inside surface to which an ultrasonic transducer is bonded can reach 400 to 600 degrees Fahrenheit. Heating to this temperature range greatly reduces bond reliability even under otherwise ideal circumstances. The bonding problem is exacerbated, however, by cyclical heating and cooling of the weld electrode.
Moreover, when high temperature epoxy is used to bond the transducer assembly to the electrode, and as potting material to hold electrical leads in place, the epoxy tends to thermally insulate the surface of the electrode that most needs to be cooled in order to prevent disbonding.
Presently, there exists no generally acceptable way of monitoring weld quality and of controlling the welding process based on monitoring information. It is thus a general practice of industry to make many more welds than are theoretically required in order to ensure the required mechanical strength in the finished assembly. Further, even when utilizing this redundant weld approach, there is no absolute assurance that projected structural integrity has been achieved in the final assembly. Thus, there exists a need for a method and apparatus for monitoring spot weld quality while the weld is being made.
It is accordingly an object of the invention to provide an ultrasonic transducer for monitoring spot weld quality which can be fitted into a commonly used spot weld electrode and which will reliably survive the environment of the electrode without degradation.
Another object of the invention is to provide an ultrasonic transducer having a minimum useful life of approximately 10,000 weld cycles.
It is a further object of the invention to provide an ultrasonic transducer which exhibits good ultrasonic signal characteristics with high signal to noise ratio, and which has minimum effect on the efficiency of water cooling of the weld electrode.
Further objects of the invention are to provide an ultrasonic transducer which is removable from the electrode, because electrode life is normally in the range of 3,000 to 5,000 weld cycles; to provide a transducer assembly which is retained to the weld electrode shank while electrode tips are being replaced; and to provide a transducer assembly which is easily removed and exchanged if damaged.